Studies of ethylene oxide adsorption on Pt-Sn alloys with TPD, HREELS, UPS, and DFT calculations

Ethylene oxide (EO) adsorption was studied on two ordered surface alloys: (2 × 2)-Sn/Pt(111) with θ_Sn = 0.25 ML, and (√3 × √3)R30°-Sn/Pt(111) with θ_Sn = 0.33 ML. Nearly all EO desorbs molecularly during TPD in peaks at 198 K for the (2 × 2) alloy and 190 K for the (√3 × √3)R30° alloy, corresponding to desorption activation energies of 12.1 and 11.6 kcal/mol, respectively. HREELS and UPS show that EO adsorbs molecularly even at low coverage through the oxygen atom with the molecular plane tilted away from the surface normal. DFT calculations provide details for the adsorption geometry: EO adsorbs without ring breaking, with oxygen bonding to Sn at a bond distance of 2.7-2.8 å and a C-O-Sn bond angle of ∼120°. XPS indicates that the EO saturation coverage is not affected by the presence of Sn in the alloys and remains the same as for pure Pt(111) at 0.25 ML. A small amount (1-2%) of EO decomposition occurs during TPD for both alloy surfaces, and DFT calculations suggest that EO transforms into a more stable metallacycle configuration with oxygen bonding to Sn prior to decomposition.